Core 2 will be responsible for measuring the concentrations and the physical and chemical properties of the particles in outdoor and indoor environments for all three epidemiologic studies. Outdoor particle concentrations will be measured for all program projects at an ambient monitoring site located near HSPH. Continuous and integrated particle measurements will be made, including mass concentration, size distribution, oxidative potential, and composition of outdoor particles. Inorganic ion, strong acidity, trace metal, and elemental and organic carbon concentrations will be measured, as will carbon monoxide (CO), ozone (O3 ), nitrogen dioxide (NO2), and sulfur dioxide (S02). In other PM2.5 concentrations will also be measured in residential and other micro- environments using our recently developed Continuous Aerosol Mass Monitor (CAMM) for measurement of + hour particle concentrations. These indoor particle measurements will be supplemented by continuous measurements of sulfate (a tracer of outdoor pollution) and CO concentrations. These measurements will make it possible to (1) relate particulate and gaseous exposures to health effects on a near-real time basis, (2) characterize the diurnal variation in indoor particle concentrations, (3) estimate the contribution of outdoor particles to indoor particulate mass concentrations, and (4) determine the importance of confounding by CO. This provides all of the information on outdoor and micro-environmental exposures. In addition, it will provide support to all projects in understanding the sources of and types of particles and relating the observed health effects to the measured particle physico-chemical properties. This Core not only provides engineering expertise and facilities for characterizing exposures, but also provides an exposure assessment framework that will integrate exposure and health effects. This exposure framework will be of paramount importance to the interpretation and synthesis of the collected data.